This invention relates to a rod of wear-resistant hard facing for application to wear or cutting surfaces such as teeth on bits for drilling oil wells or the like. A new rod and a method for manufacturing such rods are described.
Bits for drilling oil wells and other products often have a steel body which is dressed or coated with a layer of hard facing material to resist wear or provide a cutting surface which abrades rock. Conventional hard facing usually comprises particles of tungsten carbide bonded to the steel by a metal alloy. In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. Most hard facing on rock bits employs steel as the matrix, although other "brazing" alloys may also be used.
It is quite common in referring to the material in the hard facing merely as "carbide" without characterizing it as tungsten carbide, the metal carbide principally used in hard facing. Small amounts of tantalum carbide and titanium carbide (TIC/TAC) may be present along with the tungsten carbide. It will be understood that as used herein, reference merely to "carbide" means tungsten carbide, with or without small amounts of TIC/TAC.
A typical technique for applying hard facing to the steel surface is by oxyacetylene or atomic hydrogen welding. A welding rod or stick is formed of a tube of mild steel sheet enclosing a filler which is primarily carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. When using such a tube-rod, the deoxidizer alloys with the mild steel of the tube to form an alloy steel matrix. It has been desirable to use TIG welding for applying hard facing, but that has not been feasible with existing materials.
Welding rods are also made by melting the binder metal in a graphite mold along with the carbide particles to make a "cast" product. Alloy steel or brazing alloy binders may be used.
The hard facing is applied by melting an end of the rod on the surface to be hard faced. The steel tube or cast matrix melts to weld or braze to the steel substrate and provide the matrix for the carbide particles.
Three types of tungsten carbide have been employed for hard facing. Possibly the most common is crushed cast carbide. Tungsten forms two carbides. WC and W.sub.2 C and there can be an essentially continuous range of compositions therebetween. Cast carbide is typically a eutectic mixture of the WC and W.sub.2 C compounds, and as such is substoichiometric, that is, it has less carbon than the more desirable WC form of tungsten carbide. Cast carbide is frozen from the molten state and comminuted to the desired particle size.
Another type of tungsten carbide is so-called macro-crystalline tungsten carbide. This material is essentially stoichiometric WC in the form of single crystals. Most of the macrocrystalline tungsten carbide is in the form of single crystals. When larger particle sizes are examined, it is found that some bicrystals of WC are formed. Macrocrystalline WC is desirable for its toughness and stability.
The third type of tungsten carbide used in hard facing comprises cemented tungsten carbide, sometimes referred to as sintered tungsten carbide. Cemented tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Cemented tungsten carbide is made by mixing tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and "sintering" the composite at temperatures near the melting point of cobalt. The matrix melts and wets the surfaces of the tungsten carbide particles to form a composite with little if any void space. The resulting dense cemented carbide can then be comminuted to form particles of cemented tungsten carbide for use in hard facing.
Although widely used, tube-rods have certain shortcomings. The tube-rod is made by forming a tube around a powder mixture and periodically cutting the tube and crimping its ends to retain the powder mixture in place. Thermoset binder resin may also be employed for retaining the granular mixture. This is not always satisfactory and some of the hard facing product may sift out of the ends of the tube. Further, when welding, the crimped end of the tube is melted and some of the granular mixture may come out unevenly.
For such reasons, and since the tube-rod is inherently nonhomogeneous, there is often a nonuniform distribution of carbide particles in the matrix melted onto the surface. Such nonuniformity can result in erratic wear resistance. Further, the steel of the tube may not mix uniformly with the alloying ingredients in the powder within the tube, and the composition of the matrix may be nonuniform. The range of alloys available for the matrix is limited by the availability of sheet alloys for forming the tubes. Generally speaking, it is necessary to employ mild steel sheet for the tubing and try to obtain alloying by powders included with the carbide inside the tube.
Welding rods for hard facing which are made by casting the binder with the carbide particles have a different range of problems. The high temperatures of casting result in dissolution of some of the carbide in the matrix. This changes the composition of the matrix and erodes the carbide particles. The high carbon content of the matrix may be particularly troublesome since it embrittles the matrix.
It has recently been found that it is desirable to enhance the quantity of carbide relative to the binder or matrix for enhanced wear resistance in some applications. There are limits to the amount of carbide that can be applied by way of a tube since there are certain minimum thicknesses of sheet that can be used for making the tube and still maintain integrity of the tube.
It is, therefore, desirable to provide a welding rod for hard facing which can have any desired alloy composition of the matrix for the carbide particles and which can deposit the carbide particles uniformly throughout the matrix. It is desirable that the welding rod be manufactured without the costly rolls and rolling equipment required for making tube-rod. It is desirable to provide a hard facing having higher carbide content and lower matrix content than readily available with tube-rod.